In recent years, copying machines, laser printers and the like are controlled by a microcomputer (hereinafter referred to as "MC") including a central processing unit (hereinafter referred to as "CPU"), a read-only memory (hereinafter referred to as "ROM") and a random access memory (hereinafter referred to as "RAM") which is amenable to data reading and writing.
The ROM has stored therein the program to be used for the control and it is usually in the form of a mask ROM. As shown in FIG. 1, the program stored in such a ROM includes a reset-start address with a jump destination address written therein for reset-start, a routine for initialization, a routine for an interruption, a main routine for controlling a copying system or the like generally in its entirety, and subroutines for specifically controlling individual functions, etc. The power supply or reset switch, when turned on, permits the MC to read out the program first at the reset-start address (0000H address). The head address of the main routine is written in the reset-start address, and the flow of the program jumps to the main routine. Within the main routine, the initialization routine and subroutines are suitably called, and while upon request for an interruption, the interruption routine concerned is executed.
With such MCs, it is likely that a bug will be found in the program within the ROM, or it may be desired to alter the program or to add some routine thereto for a change in the specifications of the copying system or for an improvement in its function. It then becomes necessary to replace the entire memory chip, or when the ROM is in the form of a mask ROM included in a one-chip MC, the one-chip MC must be replaced in its entirety, hence a very great loss can occur.
On the other hand, it has been proposed to use as attached to an internal ROM, an external ROM wherein a program for performing a specified function is stored as a subroutine and which is available as an option as shown in FIG. 2 (Examined Japanese Patent Publication SHO 61-27783 or Unexamined Japanese Patent Publication SHO 61-23167). According to the proposal, the main routine in the internal ROM is used as such although the specified function can be added to the contemplated system, so that it is impossible to make a change in the specifications of the system if the change is relevant to the main routine or to debug the main routine.